Among positive photoresist compositions are the chemical amplification type resist compositions described, e.g., in U.S. Pat. No. 4,491,628 and European Patent 249,139. A chemical amplification type positive resist composition is a pattern-forming material in which an acid generates in exposed areas upon irradiation with a radiation such as far ultraviolet rays and this acid catalyzes a reaction that makes the areas irradiated with the actinic rays and the unirradiated areas to differ in solubility in a developing solution to thereby form a pattern on a substrate.
Examples of such resist compositions include combinations of a compound which generates an acid upon photodecomposition with an acetal or O,N-acetal compound (see JP-A-48-89003; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), with an orthoester or amidoacetal compound (see JP-A-51-120714), with a polymer having acetal or ketal groups in the backbone (see JP-A-53-133429), with an enol ether compound (see JP-A-55-12995), with an N-acyliminocarbonic acid compound (see JP-A-55-126236), with a polymer having orthoester groups in the backbone (see JP-A-56-17345), with a tertiary alkyl ester compound (see JP-A-60-3625) with a silyl ester compound (see JP-A-60-10247), and with a silyl ether compound (see JP-A-60-37549 and JP-A-60-121446). These combinations show high photosensitivity since they have a quantum efficiency exceeding 1 because of their principle.
Examples of systems which decompose upon heating in the presence of an acid to become alkali-soluble include systems comprising a combination of a compound which generates an acid upon exposure to light with an ester having a tertiary or secondary carbon (e.g., t-butyl or 2-cyclohexenyl) or with a carbonic ester compound, as described in, e.g., JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642, JP-A-5-181279, Polym. Eng. Sce., Vol.23, p.1012 (1983), ACS. Syr., Vol.242, p.11 (1984), Semiconductor World, November 1987 issue, p.91, Macromolecules, Vol.21, p.1475 (1988), and SPIE, Vol.920, p.42 (1988); systems comprising a combination of the acid-generating compound with an acetal compound, as described in, e.g., JP-A-4-219757, JP-A-5-249682, and JP-A-6-65332; and systems comprising a combination of the acid-generating compound with a t-butyl ether compound, as described in, e.g., JP-A-4-211258 and JP-A-6-65333.
These systems employ as the main component a resin having a poly(hydroxystyrene) backbone, which shows low absorption mainly in a wavelength region including 248 nm. Consequently, these systems have high sensitivity and high resolution and are capable of forming a satisfactory pattern when a KrF excimer laser is used as a light source for exposure.
However, the degree of integration in integrated circuits is increasing more and more, and it has become necessary to form an ultrafine pattern having a line width of 0.5 .mu.m or smaller in the production of semiconductor substrates for ULSIs and the like.
Known as one means for attaining finer patterns is to use an exposure light having a shorter wavelength in resist pattern formation. This can be explained with the Rayleigh's equation which represents the resolution (line width) R of an optical system: EQU R=k.multidot..lambda./NA
(wherein .lambda. is the wavelength of exposure light, NA is the numerical aperture of a lens, and k is a process constant). This equation shows that a higher resolution, i.e., a smaller value of R, can be attained by using an exposure light having a shorter wavelength .lambda..
For example, the i-ray (365 nm) emitted from a high-pressure mercury lamp has been used so far in the production of DRAMs having degrees of integration of up to 64 megabits. For processes for mass-producing 256-megabit DRAMs, investigations are being made on the employment of KrF excimer laser light (248 nm) as an exposure light substitute for i-ray. For use in producing DRAMs having a degree of integration of 1 gigabit or higher, exposure lights having even shorter wavelengths are being investigated and use of ArF excimer laser light (193 nm), F.sub.2 excimer laser light (157 nm), X-rays, electron beams, and the like is thought to be effective [see Takumi Ueno et al., "Tanpacho Fotorejisuto Zairyo--ULSI Nimuketabisaikako--(Short-wavelength Photoresist Materials--Fine Processing toward ULSIs--)", Bunshin Shuppan, 1988).
In the case of using a light source emitting a shorter-wavelength light, e.g., an ArF excimer laser (193 nm), as a light source for exposure, even the chemical amplification type systems described above have been insufficient because compounds having aromatic groups intrinsically show intense absorption in a region including 193 nm.
Although use of a poly(meth)acrylate as a polymer showing reduced absorption in a wavelength region including 193 nm is described in J. Vac. Sci. Technol., B9, 3357 (1991), this polymer has had a problem that it is inferior to conventional phenolic resins having aromatic groups in resistance to the dry etching generally conducted in semiconductor production processes.
In contrast, it has been reported in Proc. of SPIE, 1672, 66 (1992) that a polymer having alicyclic groups is comparable in dry etching resistance to the aromatic polymers and shows reduced absorption in a region including 193 nm. This report has led to recent enthusiastic investigations on the utilization of that kind of polymer. Examples of that polymer are described in, e.g., JP-A-4-39665, JP-A-5-80515, JP-A-5-265212, JP-A-5-297591, JP-A-5-346668, JP-A-6-289615, JP-A-6-324494, JP-A-7-49568, JP-A-7-185046, JP-A-7-191463, JP-A-7-199467, JP-A-7-234511, JP-A-7-252324, and JP-A-8-259626. However, these polymers do not always have sufficient dry etching resistance and are disadvantageous also in that the synthesis thereof necessitates many steps.
The larger the dissolution rate between before and after exposure in a chemical amplification type resist, the higher the resolution of the resist. Consequently, in chemical amplification type resists of the type in which a resin and a dissolution inhibitor are changed in solubility, higher resolution is obtained when the dissolution inhibitor has a higher dissolution inhibitive effect. Although there has been almost no dissolution inhibitor which is applicable also to far ultraviolet light having a wavelength as short as 220 nm or below, a photoresist composition has been disclosed which contains a dissolution inhibitor comprising a specific compound and a photo-acid generator which generates an acid upon light irradiation (see JP-A-9-265177).
Furthermore, a composition comprising a substituted androstane, a radiation-sensitive acid generator, and a copolymer binder has been disclosed as a radiation-sensitive resist composition for far ultraviolet rays which has improved sensitivity and resolution and which, when used as a resist film having a thickness of 1 .mu.m and exposed to ultraviolet rays having a wavelength as short as 193 nm, can sufficiently form an image even at a radiation dose below about 15 mJ/cm.sup.2 (see JP-A-8-15865).
However, these compositions for short-wavelength light exposure have had problems of considerable pattern cracking, poor pattern/substrate adhesion, fine-pattern falling, etc.
A radiation-sensitive resin composition suitable for use as a chemical amplification type photoresist has been disclosed in which the transmission of far ultraviolet rays can be sufficiently controlled and which is considerably reduced in standing wave marks and halation and satisfactory in developability, pattern shape, etc. and has sufficient resistance to dry etching. This radiation-sensitive resin composition comprises (A) a resin which releases protective groups by the action of an acid to become alkali-soluble, (B) a radiation-sensitive acid generator, and (C) at least one compound selected from (i) alicyclic low-molecular compounds having one or more hydrophilic functional groups and from 5 to 25 carbon atoms and (ii) naphthalenic low-molecular compounds having from 10 to 40 carbon atoms (see JP-A-9-274318).
However, the above radiation-sensitive resin composition which transmits far ultraviolet rays still has insufficient sensitivity when exposed to a short-wavelength light, e.g., ArF excimer laser light (193 nm). Since ArF excimer laser light has a high energy, exposure with an ArF excimer laser over a prolonged period poses a problem that the life of the expensive exposure apparatus becomes shorter due to the exposure energy. Consequently, use of a resist composition having insufficient sensitivity results in a longer exposure time and hence leads to a shorter life of the exposure apparatus. The prior art composition described above further has problems of, e.g., the occurrence of development defects, poor pattern/substrate adhesion, and fine-pattern falling.